1. Field of the Invention
The present invention relates to a laminated dielectric element such as a laminated ceramic capacitor, a laminated piezo-electric actuator or the like and to a method of producing the same.
2. Description of the Related Art
Laminated dielectric elements obtained by alternately laminating dielectric ceramic layers made of a PZT-type material having excellent dielectric properties and base metal electrode layers of a base metal such as copper or the like, have heretofore been widely utilized for capacitors and actuators.
The laminated dielectric elements are usually produced through a plurality of steps as described below.
First, a green sheet is prepared by using a ceramic material such as PZT, and a paste electrode material of a metal oxide is applied onto the green sheet by screen-printing or the like method. Next, the green sheets on which the paste electrode material is applied are laminated to prepare a laminate thereof, which is then dewaxed.
Then, the laminate after being dewaxed is heated in a heating furnace under reducing conditions to reduce the metal oxide in the paste electrode material and to form metal electrode layers having electric conduction (step of reducing the electrodes). Thereafter, the laminate is sintered to increase the density of the ceramic material to finally obtain a laminated dielectric element (step of sintering).
In the above step of sintering, however, the paste electrode material and the ceramic material require conflicting atmospheric conditions. That is, the ceramic material such as PZT which is an oxide can be favorably sintered in an oxidizing atmosphere, whereas the paste material is better sintered in a reducing atmosphere to maintain electric conduction obtained through the step of reducing the electrodes.
Therefore, if the step of sintering is conducted in an oxidizing atmosphere to fire the ceramic material to a sufficient degree, then the metal electrode layers of copper or the like, that are reduced in advance in the step of reducing the electrodes, are oxidized again causing the electric conduction to be decreased. If the sintering is conducted in a reducing atmosphere, on the other hand, the ceramic material is reduced, and properties of the laminate after sintering deteriorate.
In order to solve the above problem, there has been proposed a method according to which an electrically conducting paste is reduced in an atmosphere containing a hydrogen gas and, in a subsequent step of sintering, the laminate is sintered in an atmosphere in which the oxygen partial pressure is controlled to lie in a particular range (see Japanese Unexamined Patent Publication (Kokai) No. 5-82387).
There has further been proposed a method in which an electrically conducting paste is reduced at a temperature lower than the sintering temperature, and in a subsequent step of sintering, the laminate is sintered in an atmosphere in which the O2 partial pressure is controlled to lie within a particular range by using an N2—H2—H2O mixed gas (see Japanese Unexamined Patent Publication (Kokai) No. 7-34417).
According to the above-mentioned conventional methods, the density of the ceramic material can be increased almost without oxidizing the metal electrodes of copper or the like reduced in the step of reducing the electrodes in spite of having conducted the step of sintering.
In the above step of reducing the electrodes, however, there exist two reactions, i.e., a reducing reaction based on a theory of chemical equilibrium and a reducing reaction based on a theory of reaction rate, in which the above conventional methods are not suitable.
Let the case now be considered of producing the device by using a conducting paste containing, for example, a Cu oxide and a ceramic material of PZT material. Then, in the step of reducing the electrodes, there exist a Cu oxide-reducing reaction based on the theory of chemical equilibrium (2CuxO2xCu+O2) and a Cu oxide-reducing reaction based on the theory of reaction rate by hydrogen gas (CuxO+H2xCu+H2O).
According to the reaction based on the theory of reaction rate between the above two reactions, the Cu oxide is not reduced but remains in the conducting paste when the concentration or amount of hydrogen gas is small. When the concentration or amount of hydrogen gas is large, on the other hand, the Cu oxide is reduced and the PZT is reduced, too, in the ceramic material, releasing Pb and the like from the composition, which is not desirable.
When the Cu oxide is not reduced but remains as described above, Cu2O which is a Cu oxide that remains reacts with PbO and the like remaining in small amounts in the ceramic material such as PZT to form a eutectic material of liquid phase in the step of sintering, and this liquid phase diffuses in the ceramic material. Therefore, there is a problem of a decrease in the insulating resistance after sintering. Further, a portion where the liquid phase has occurred tends to be contracted as compared to the surrounding areas, giving rise to the problem of delamination during the sintering.
When Pb is released from the composition as a result of reducing the PZT, on the other hand, Pb that is released and Cu formed by reduction in the step of reducing the electrodes may form a eutectic material of liquid phase at temperatures of not lower than 327° C. which is a eutectic point thereof. As a result, in the above sintering step of effecting the heat treatment at a high temperature, the metal electrodes may dissolve. Further, since the portion where the liquid phase is formed tends to be more contracted than the surrounding areas, there is a problem of delamination occurring during the step of sintering.
The occurrence of delamination degrades important characteristics of the laminated dielectric element such as Young's modulus, insulation resistance, etc.